A typical image sensor senses light by converting impinging photons into electrons that are integrated (collected) in sensor pixels. After completion of integration cycle charge is converted into a voltage that is supplied to output terminals of the sensor. In CMOS image sensors the charge to voltage conversion is accomplished directly in the pixel itself and the analog pixel voltage is transferred to the output through various pixel addressing and scanning schemes. The pixels have incorporated in them a buffer amplifier, typically a source follower, which drives the sense lines that are connected to the pixels by suitable addressing transistors. The analog pixel signal can also be converted into a digital signal format on its way to the output. The digital signals are less susceptible to distortions, attenuation, and noise pickup and for this reason it is advantageous to make the conversion to digital format at the very beginning of the signal processing chain. Examples of sensors with signal conversion to digital format directly in the pixel itself can be found in U.S. Pat. No. 6,229,133 to Hynecek and in U.S. Pat. No. 5,461,425 to Fowler at al. However, this approach has also its own problems. The pixel with digital conversion incorporates many transistors and as a result has smaller aperture efficiency and sensitivity. Another problem is the A/D converter itself. The conversion typically relies on some reference voltage or a threshold to which the pixel output is compared. Since the pixel buffer amplifier has its own DC output offset level, that can vary from pixel to pixel, and the reference threshold of the A/D converter can also vary, the resulting digital output may be very non-uniform. This problem is tackled by using various analog readout schemes, such as the Correlated Double Sampling (CDS) that is applied before the A/D conversion. The CDS minimizes pixel non-uniformities by reading the pixel signal twice, once with charge and once without it. The difference is then free of the pixel DC level variations and the A/D conversion can take place. The CDS concept, however, does not eliminate non-uniformities in the column readout circuits even if the column readout is digital. The A/D converter threshold uniformity problems are minimized by using complicated self-calibrating and auto zeroing techniques as mentioned for example in the article: “A 1¼ inch 8.3 M Pixel Digital Output CMOS APS fro UDTV Applications” by: I. Takayanagi at al. published in: “Digest of Technical Papers 2003 IEEE International ISSCC Conference, pp. 216”. Unfortunately such complicated systems can be used only in the array columns, since it would not be practical to integrate them into the pixels themselves. Such circuits also consume power and occupy a significant chip area thus contributing to increased cost of these sensors.
The sensor pixel non-uniformities, the column-to-column non-uniformities, and the A/D threshold variations thus represent persistent problem in CMOS image sensors that is not easily solved.